The awareness about global environmental problems has been raised and, among them, exhaustion of fossil raw materials and petroleum resources and increases in carbon dioxide have been perceived as problems. Consequently, research and development on biodegradable resins, e.g., aliphatic polyesters, and resins synthesized from plants serving as raw materials have been conducted actively. Among the aliphatic polyesters, in particular, a polylactic acid having excellent moldability is noted as a plant-derived resin formed by using lactic acid, which is produced from grain resources, e.g., corn, through fermentation, as a raw material.
However, polylactic acid has drawbacks of being rigid and brittle and, in addition, the crystallization rate is small and the heat resistance is low. Therefore, there is a limitation on development of uses. In particular, as for a polylactic acid amorphous molded body, the softening point is lower than 60° C. and, thereby, such a problem has been pointed out that whitening, deformation, and the like occur easily under a usual use environment.
Furthermore, if an improvement of the heat resistance of the polylactic acid by increasing the crystallinity through a heat treatment (anneal) is intended, there is usually a problem in that crystals (for example, spherulite) having sizes nearly equal to or larger than a wavelength of light and causing scattering of light grow rapidly, so as to become opaque.
In order to solve the above-described problems, many attempts to improve the heat resistance and the transparency by adding various additives to the polylactic acid have been made.
Patent Document 1 describes that addition of phosphoric acid ester metal salts, hydrous magnesium silicate, and the like serving as nucleators is effective. However, in the case where such a nucleator is used, there is a drawback that the transparency is impaired. Moreover, generally used talc is within the range of practical use from the viewpoint of merely the crystallization rate. However, for that purpose, in many cases, it is required that the amount of addition is 1% or more. Consequently, there is a drawback that the transparency, which is a characteristic intrinsic to the polylactic acid, is impaired.
Patent Document 2 describes a method, in which as for the nucleator, at least one type selected from aliphatic carboxylic acid amides, aliphatic carboxylic acid salts, aliphatic alcohols, and aliphatic carboxylic acid esters is added as a transparent nucleator. However, in this case, the haze is 6.5% at a crystallinity of 33% and a result exhibiting combination of sufficient crystallinity and transparency has not been obtained.
Patent Document 3 describes a method, in which a polylactic acid formed by using a compound having a specific functional group as an initiator and an inorganic filler are used. Although sliding characteristics are improved by this method, it is not possible to ensure the transparency because the inorganic filler is added.
On the other hand, Patent Document 4 describes block copolymers of polyolefins, e.g., polyethylene and polypropylene, and polylactic acids. However, there is no description on the transparency. In particular, the transparency at a high crystallinity is not touched on. Furthermore, the olefin based block copolymers used in Patent Document 4 is not easily industrially produced because complicated and high level reactions are used, and the use as general-purpose polymers is unfavorable from the viewpoint of the cost.
Patent Document 5 describes an aliphatic polyester block copolymer having a polyolefin segment and an aliphatic polyester segment. However, the block copolymer is produced by the method in Patent Document 4 described above and, therefore, as in the case of the olefin based block copolymers in Patent Document 4, it is not said that there is a cost advantage.
In addition, Patent Document 5 describes a method, in which a compound having a hydroxyl group and an unsaturated bond, e.g., 2-hydroxyethyl methacrylate (HEMA), is radically reacted with a polypropylene so as to obtain a copolymer, in which HEMA and oligomers thereof are grafted on a polypropylene chain, and thereafter, a polylactic acid segment is introduced through the use of the hydroxyl group. However, it is generally known that in the case where this method is applied to a polyethylene, radical reactions occur at various places of the ethylene chain and, thereby, cross-linking reaction between polyethylene chains occurs at the same time and gelation occurs easily.
Furthermore, in any case, it is usually unavoidable that homopolymers of a grafting agent are generated through side reactions.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-192884    Patent Document 2: Japanese Unexamined Patent Application Publication No. 9-278991    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-285121    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2001-270924    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2007-177039